Put half the brownie mix into a large bowl. Add the sweetened condensed milk and mix until thoroughly combined.

Pour the heavy cream into the bowl of a stand mixer. Beat with whisk attachment until stiff peaks form, then add the vanilla and beat to combine. Carefully fold the whipped cream and remaining brownie mix into the condensed milk mixture with a spatula. Continue gently folding until the two are well mixed (this may take a few minutes).

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